Adaptive Immune System Components

The adaptive immune system is crucial for defending the body against specific pathogens. It relies on T and B lymphocytes, which work together to recognize, attack, and remember invaders, ensuring a stronger response during future encounters.

1. T lymphocytes (T cells)

   • Key players in cell-mediated immunity, responsible for directly attacking infected or cancerous cells.
   • Differentiate into various subtypes, including helper T cells (CD4+) and cytotoxic T cells (CD8+).
   • Require antigen presentation by MHC molecules to become activated.

2. B lymphocytes (B cells)

   • Central to humoral immunity, primarily responsible for producing antibodies.
   • Differentiate into plasma cells upon activation, which secrete large quantities of antibodies.
   • Can also form memory B cells for long-term immunity.

3. Antibodies (immunoglobulins)

   • Proteins produced by B cells that specifically bind to antigens, neutralizing pathogens or marking them for destruction.
   • Exist in different classes (IgG, IgA, IgM, IgE, IgD), each with distinct functions and locations in the body.
   • Play a crucial role in opsonization, complement activation, and immune response regulation.

4. Major Histocompatibility Complex (MHC) molecules

   • Cell surface proteins that present peptide fragments (antigens) to T cells, essential for T cell activation.
   • Two main classes: MHC Class I (present on all nucleated cells, recognized by CD8+ T cells) and MHC Class II (present on APCs, recognized by CD4+ T cells).
   • Critical for self/non-self recognition and immune tolerance.

5. Antigen-presenting cells (APCs)

   • Specialized cells (e.g., dendritic cells, macrophages, B cells) that capture, process, and present antigens to T cells.
   • Express MHC Class II molecules to activate CD4+ T helper cells.
   • Play a vital role in initiating and regulating adaptive immune responses.

6. Cytokines

   • Signaling molecules that mediate communication between immune cells, influencing their activity and behavior.
   • Include various types such as interleukins, interferons, and tumor necrosis factors, each with specific roles in immune regulation.
   • Essential for coordinating the immune response, promoting cell growth, and directing cell migration.

7. Memory cells

   • Long-lived cells that arise after an initial immune response, providing rapid and robust protection upon re-exposure to the same antigen.
   • Include both memory T cells and memory B cells, ensuring a quicker and more effective response in future infections.
   • Form the basis of immunological memory, which is the principle behind vaccinations.

8. Plasma cells

   • Fully differentiated B cells that produce and secrete large amounts of antibodies specific to an antigen.
   • Short-lived but crucial for the immediate immune response during an active infection.
   • Contribute to the humoral immune response by providing antibodies that neutralize pathogens.

9. T cell receptor (TCR)

   • A molecule found on the surface of T cells that recognizes and binds to specific antigens presented by MHC molecules.
   • Composed of two chains (alpha and beta or gamma and delta) that create a unique binding site for each T cell.
   • Essential for T cell activation and subsequent immune response.

10. B cell receptor (BCR)

    • A membrane-bound immunoglobulin on B cells that recognizes specific antigens, initiating B cell activation.
    • Functions similarly to antibodies but is attached to the B cell surface.
    • Plays a critical role in the differentiation of B cells into plasma cells and memory B cells upon antigen encounter.